Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G. The 3GPP recently launched a study item “Evolved UTRA and UTRAN”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning and reduce user and operator costs. It is generally assumed that there will be a convergence toward the use of Internet Protocols (IP), and all future services will be carried on top of IP. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain.
The main objectives of the evolution are to further improve service provisioning and reduce user and operator costs as already mentioned. More specifically, some key performance and capability targets for the long-term evolution (LTE) are inter alia:                significantly higher data rates compared, to HSDPA and HSUPA (envisioned are target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink)        high data rates with wide-area coverage        significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup), and        threefold system capacity compared to current standards.        
One other key requirement of the long-term evolution is to allow for a smooth migration to these technologies.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single carrier transmission combined with FDMA and dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR) compared to multi-carrier signals (such as OFDMA), the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). In each time interval, Node B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-ell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra/cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one transmission time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
The frequency resource can either be in a localized or distributed spectrum as illustrated in FIG. 3 and FIG. 4. As can be seen in FIG. 3, localized single-carrier is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths of a localized single-carrier signal.
On the other hand, as can be seen in FIG. 4, distributed single-carrier is characterized by the transmitted signal having a non-continuous (“comb-shaped”) spectrum that is distributed over system bandwidth. Note that, although the distributed single-carrier signal is distributed over the system bandwidth, the total amount of occupied spectrum is, in essence, the same as that of localized single-carrier. Furthermore, for higher/lower symbol rate, the number of “comb-fingers” is increased/reduced, while the “bandwidth” of each “comb finger” remains the same.
At first glance, the spectrum shown in FIG. 4 may give the impression of a multi-carrier signal where each comb-finger corresponds to a “sub-carrier”. However, from the time-domain signal-generation of a distributed single-carrier signal, it should be clear that what is being generated is a true single-carrier signal with a corresponding low peak-to-average power ratio.
The key difference between a distributed single-carrier signal vs. a multi-carrier signal, such as e.g. OFDM, is that, in the former case, each “sub-carrier” or “comb finger” does not carry a single modulation symbol. Instead each “comb-finger” carries information about all modulation symbol. This creates a dependency between the different comb-fingers that leads to the low-PAPR characteristics. It is the same dependency between the “comb fingers” that leads to a need for equalization unless the channel is frequency-non-selective over the entire transmission bandwidth. In contrast, for OFDM equalization is not needed as long as the channel is frequency-non-selective over the sub-carrier bandwidth.
Distributed transmission can provide a larger frequency diversity gain than localized transmission, while localized transmission more easily allows for channel-dependent scheduling. Note that, in many cases the scheduling decision may decide to give the whole bandwidth to a single UE to achieve high data rates.
Uplink Scheduling Scheme
The uplink scheme should allow for both scheduled (Node B controlled) access and contention-based access. In case of scheduled access the UE is dynamically allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission.
Some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled.
For the scheduled access Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. For example, the scheduler determines                a which UE(s) is (are) allowed to transmit,        which physical channel resources (frequency),        for how long the resources may be used (number of sub-frames)        Transport format (e.g. Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the UE via a scheduling grant sent on the downlink control channel. In LTE, for simplicity this channel is also referred to as LTE_HS_SCCH (Long Term Evolution—High Speed—Shared Control CHannel). A scheduling grant message contains at least information on which part of the frequency band the UE is allowed to use, whether localized or distributed spectrum should be used, the validity period of the grant, and the maximum data rate. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme.
Uplink data transmissions are only allowed to use the time-frequency resources assigned to the UE through the scheduling grant. If the UE does not have a valid grant, it is not allowed to transmit any uplink data. Unlike in HSUPA, where each UE is always allocated a dedicated channel there is only one uplink data channel shared by multiple users (UL SCH—UpLink Shared CHannel) for data transmissions. Furthermore, there is only one mode of operation for the uplink data access in LTE, the above described scheduled access, i.e. unlike in HSUPA where both scheduled and autonomous transmissions are possible.
To request resources, the UE transmits a resource request message to the Node B. This resources request message could for example contain information on the amount of data to transmit, the power status of the UE and some Quality of Services (QoS) related information. This information, which will be referred to as scheduling information, allows Node B to make an appropriate resource allocation.
Resource requests are transmitted using the contention-based access compared to the above described scheduled access. However, if the UE already has a valid grant, e.g., if a data transmission is ongoing, the resource requests updates can be transmitted using the granted resources, e.g., as part of MAC headers or MAC control PDU. Contention-based access can be seen as a special case of the normal scheduled access, where Node B assigns a physical resource to one user. In case of contention-based access a physical resource (subcarriers) is assigned/shared to multiple UEs for uplink transmission. The allocation for the contention-based channel, also referred to as random access channel, is for example signaled on a broadcast channel, so that all UEs in a ell have access to this area.
FIG. 5 illustrates an exemplary allocation for contention-based access. The bandwidth of the random access channel may for example depend on the estimated number of simultaneous accessing users and on the size of the messages transmitted on the channel. In the depicted example, the random access channel is allocated in a TDM fashion, one out of X subframes forming a frame is reserved for contention-based access over the entire frequency band. However it's also possible to allocate only part of the total bandwidth for random access in a distributed spectrum, in order to benefit further from frequency diversity.
Since the access is not scheduled, there is a probability, that multiple UEs access the random access channel simultaneously, leading to collisions. UE-specific scrambling and processing gain can be used in order to separate the various transmissions. The contention-based access should be only used for requesting resources in case UE has no valid grant assigned or for the initial access (going from idle to connected mode).
Channel-dependent scheduling should be also supported by the uplink-scheduling scheme in LTE. However, since there is no transmission from non-scheduled UEs, it's not straightforward.
The scheduler, typically located in the Node B for LTE, needs to know the users uplink channel status before allocating resources by means of a channel-dependent scheduling algorithm. Therefore UE may transmit pilot bits, which are known at the receiver side, prior to the data transmission to support channel-dependent scheduling. Node B can consider the measured C/I ratio (Carrier-to-Interference ratio) of the pilots bits for the resource allocation.
Scheduling Related Control Signaling
The Node B controlled scheduled access is based on uplink and downlink control signaling together with a specified UE behavior with respect to the control signaling.
In the downlink a resource allocation message is transmitted from Node B to the UE indicating the physical resources (time/frequency resource) assigned to this user. As already mentioned above this allocation message, also referred to as scheduling grant, contains information on the identification of the user the resource allocation is addressed to, the reserved physical resource (time/frequency resource), some information on the maximum data rate, modulation and coding scheme and also probably some HARQ related information (redundancy version).
In the uplink UE sends a scheduling request to the Node B when data for uplink transmission is available in the buffer. The scheduling request message contains information on the UE status, e.g. buffer status, QoS related information, power headroom information. This in turn allows Node B to make an appropriate allocation of resources considering also QoS requirements of the data to be transmitted.
In parallel to the actual uplink data transmission, UE signals data related control signaling, providing information on the current data transmission similar to the E-DPCCH signaling in UMTS Release 6 (HSUPA). This control signaling contains information on the chosen transport format (TFCI), which is used for decoding the data transmission at Node B, and some HARQ related information, e.g. Redundancy version, HARQ process ID and NDI (New Data Indicator). The exact information depends obviously on the adopted HARQ protocol. For example in a synchronous HARQ protocol there is no need to signal the HARQ process ID explicitly.
Uplink Timing
To ensure orthogonality in the uplink, all UE transmissions must be time aligned at the Node B within the cyclic prefix. This is implemented by the Node B measuring the timing accuracy in a received signal and, based on the timing accuracy, transmitting a timing adjustment command to the UE. The timing adjustment command is sent as control information using the downlink SCCH. Note that a UE not actively transmitting may be out-of-sync, which needs to be accounted for in the initial random access. This timing control information commands UE to advance or retract the respective transmit timing. Two alternatives for the timing control commands are currently considered:                Binary timing-control commands implying forward/backward of the transmit timing a certain step size x μs [x TBD] transmitted with a certain period y μs [y TBD].        Multi-step timing-control commands being transmitted on the downlink on a per-need basis.        
As long as a UE carries out uplink data transmission, the received signal can be used by Node B to estimate the uplink receive timing and thus as a source for the timing-control commands. When there is no data available for uplink, the UE may carry out regular uplink transmissions (uplink synchronization signals) with a certain period, to continue to enable uplink receive-timing estimation and thus retain uplink time alignment. In this way, the UE can immediately restart uplink-orthogonal data transmission without the need for a timing re-alignment phase.
If the UE does not have uplink data to transmit for a longer period, no uplink transmission should be carried out. In that case, uplink time alignment may be lost and restart of data transmission must then be preceded by an explicit timing-re-alignment phase to restore the uplink time alignment.
Efficient scheduling in an orthogonal uplink radio access requires Node B to rapidly allocate resources, e.g. frequency/time symbols, among UEs having data for transmission thereby meeting the QoS requirements of the corresponding data. Another demand on the scheduling scheme is the support of channel-dependent scheduling in order to further improve the efficiency, e.g. system throughput. Hence, a mechanism is required for UE to request resources.
This resource request message transmitted by the UEs to request uplink resources typically contains very detailed information on the UE status, e.g. buffer status, QoS parameter and power headroom within its scheduling information. The scheduling information needs to be very precise in LTE UL in order to enable Node B to make an exact and efficient resource allocation. Therefore the message size is supposed to be much longer compared to HSUPA, where Scheduling Information only comprised 18 bits. Since UE hasn't been assigned any resources at the first step, scheduling information is sent on a contention-based access channel.
As indicated above, the scheduling information is sent on a contention-based access to the scheduler. As a consequence in order to keep the collision probability at a sufficient low level, the contention-based channel will consume a relatively large amount of resources. This may lead to an inefficient usage of the uplink resources, e.g. less bandwidth can be spent for the scheduled access. Since the size of the scheduling information message is rather long collisions may lead to an increased delay in transmission of the scheduling information, which will hence delay the whole scheduling procedure. Short message sizes are in general preferable in a contention-based access. In case the transport block size for a message transmitted on a contention-based access is fixed, the error-protection could be increased for smaller message sizes, e.g. more redundancy bits within the transport block. When transport block size depends on the message size, e.g. coding rate is fixed, the collision probability is smaller in case of smaller transport block sizes.
Another drawback of conventional scheduling schemes may be that reference signals, required for the support of channel-dependent scheduling, are only transmitted once. However the channel may change significantly for one user within the time instance of sending the reference signals and the actual resource allocation for this user. Node B could for example schedule other user, which may have a higher priority or better channel conditions, before assigning resources to this user. Therefore the channel information may not be up-to-date, which would lead to an inappropriate MCS selection.